DHCP clients that are located across a router from a DHCP server require the router be configured to forward DHCP traffic to a DHCP server that is located on a remote subnet. This DHCP traffic is broadcast traffic. Routers do not typically forward broadcast traffic unless they are configured to do this. A network router can be a hardware-based router, such as those manufactured by the Cisco Corporation, or a software-based router, such as Microsoft Routing and Remote Access Services (RRAS). In either case, you must configure the router to relay DHCP traffic to designated DHCP servers. The IP addresses of the DHCP server are configured on the router on a per-interface basis by using IP helper functionality. Or, for RRAS, the IP addresses are configured by using the DHCP Relay Agent. The following example shows an IP helper configuration: For more information about how to configure DHCP servers on third-party router products, see the appropriate configuration documentation that is included with the product.
How to configure the DHCP Relay Agent on RRAS in Microsoft Windows NT 4.0To configure the DHCP relay agent in Windows NT 4.0, follow these steps:
- Click Start, point to Programs, point to Administrative Tools, and then click Routing and RAS Admin.
- Expand the IP Routing folder, right-click Summary, and then click Add Routing Protocol.
- Click DHCP Relay Agent in the list, and then click OK.
- Right-click DHCP Relay Agent, and then click Configure DHCP Relay Agent.
- A dialog box appears in which you can type the IP addresses of the DHCP servers. Because this is a PXE client that must communicate with an RIS server, you also have to add the IP address of the RIS server. This enables the relay agent to forward DHCP broadcast traffic to both the DHCP servers and to the RIS server concurrently.
- After you add all the required IP addresses, select an interface in which to perform the relay function. To do this, right-click DHCP Relay Agent, and then click Add interface.
- A dialog box that lists all available interfaces on the RRAS server is displayed. Select the interface that is located on the same subnet as the PXE client, accept the default values (you may have to change the hop count, depending on the location of the DHCP server and the RIS server, or both), and then click OK.
- Using Network Monitor to analyze DHCP traffic, check that the giaddr field in the DHCP packets is the IP address of the relay interface on the RRAS server:
- Check the number of Requests Received and Replies Received in the right pane of the RRAS Administrator interface that is associated with the relay interface.
The DHCP server and the RIS server both receive the Discover packets, and both reply with an Offer packet. The Offer packet of the DHCP server includes the PXE client IP address and any additional options that are configured (for example, the router, the DNS server, and the WINS server): The Offer packet of the RIS server includes only the RIS server IP address, for which the client sends an ARP request in order to continue with the software installation process:
How to configure the DHCP Relay Agent in Microsoft Windows 2000 and in Windows Server 2003The process for configuring the DHCP Relay Agent in Windows 2000 and in Windows Server 2003 resembles the process that is used in Windows NT 4.0. To do this, follow these steps:
- Click Start, point to Programs or All Programs, point to Administrative Tools, and then click Routing and Remote Access.
- If you have already installed RRAS, right-click General under IP routing, and then click New Routing Protocol.
NOTE: From this point the installation and configuration of the DHCP relay agent is the same as the process in Windows NT 4.0. However, the display may vary slightly in appearance.
- After the PXE client is configured to have an IP address, it continues its conversation with the RIS server by sending another DHCP Request packet:
- The RIS server sends an Ack packet that contains the configuration information that the PXE client has to continue with the installation process
- Normal DHCP conversations between a DHCP server and a client occur on UDP Ports 67 (server) and 68 (client). The conversations between a PXE client and a RIS server during an installation occur on UDP Ports 68 (client) and 4011 (server).